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Make fast real factorial more generic.

This new version allows to compute odd and even double factorials
without harming performance for the standard factorial.
legacy
Sam Hocevar преди 7 години
родител
ревизия
80823faac6
променени са 1 файла, в които са добавени 10 реда и са изтрити 4 реда
  1. +10
    -4
      src/math/real.cpp

+ 10
- 4
src/math/real.cpp Целия файл

@@ -861,16 +861,22 @@ template<> real pow(real const &x, real const &y)
return is_odd ? exp(y * log(-x)) : -exp(y * log(-x));
}

static real fast_fact(unsigned int x)
/* A fast factorial implementation for small numbers. An optional
* step argument allows to compute double factorials (i.e. with
* only the odd or the even terms. */
static real fast_fact(unsigned int x, unsigned int step = 1)
{
real ret = real::R_1();
unsigned int start = (x + step - 1) % step + 1;
real ret = start ? real(start) : real(step);
uint64_t multiplier = 1;

for (unsigned int i = 1, exponent = 0;;)
for (unsigned int i = start, exponent = 0;;)
{
if (i++ >= x)
if (i >= x)
return ldexp(ret * multiplier, exponent);

i += step;

/* Accumulate the power of two part in the exponent */
unsigned int tmp = i;
while ((tmp & 1) == 0)


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